Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.

white hole

WHITE Hole

In astrophysics, a white hole is the hypothetical time reversal of a black hole. While a black hole acts as an attractor, drawing in any matter that crosses the event horizon, a white hole acts as a source that ejects matter from its event horizon. The sign of the acceleration is invariant (unchanged) under time reversal, so both black and white holes attract matter. The only potential difference between them is in the behavior at the horizon.

Black hole event horizons can only “suck up” matter, while white hole horizons ostensibly recede from any incoming matter at the local speed of light, so that the infalling matter never crosses. The infalling matter is then scattered and re-emitted at the death of the white hole, receding to infinity after having come close to the final singular point where the white hole is destroyed. The total proper time until an infalling object encounters the singular endpoint is the same as the proper time to be swallowed by a black hole, so the white hole picture does not say what happens to the infalling matter. Ignoring the classically unpredictable emissions of the white hole, the white hole and black hole are indistinguishable for external observers.

In quantum mechanics, the black hole emits Hawking radiation, and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium. This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking’s semi-classical argument is reproduced in a quantum mechanical AdS/CFT treatment, where a black hole in anti-de Sitter space is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.

Wormhole

WORM Hole

In physics and fiction, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a ‘shortcut’ through space and time. Although they are very popular in science fiction, there is no actual evidence that they exist. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2-D) surface (see illustration, right). Now, if we ‘fold’ this surface along a (non-existant) 3rd dimension, it allows us to picture a wormhole ‘bridge’. (Please note, though, that this image is merely a visualization displayed to convey an essentially unimaginable structure existing in 4 or more dimensions.) A wormhole is, in theory, much like a tunnel with two ends each in separate points in space-time.

There is no observational evidence for wormholes, and, although wormholes are valid solutions in general relativity, this is only true if exotic matter can be used to stabilize them. Even if the wormhole is stabilized, the slightest fluctuation in space would collapse it. If such exotic matter — that is, matter with negative mass — does not exist, all wormhole-containing solutions to Einstein’s field equations are vacuum solutions, which require an impossible vacuum, free of all matter and energy. There is no evidence or experimental suggestion that wormholes do exist, except as predictions of certain (exotic) physical models. Wormholes allowed by current physical theories might arise spontaneously, but would vanish nearly instantaneously, and would likely be undetectable.